Parton Content Research Articles

Has the QCD renormalization-group-improved parton content of virtual photons been observed?

It is demonstrated that present $e^+e^-$ and DIS ep data on the structure of the virtual photon can be understood entirely in terms of the standard `naive' quark--parton model box approach. Thus the QCD renormalization group (RG) improved parton distributions of virtual photons, in particular their gluonic component, have not yet been observed. The appropriate kinematical regions for their future observation are pointed out as well as suitable measurements which may demonstrate their relevance.

Erratum: Radiatively generated parton distributions of real and virtual photons [Phys. Rev. D60, 054019 (1999)

The parton content of real (P^2=0) and virtual (P^2\neq 0) transverse photons \gamma(P^2) is expressed in terms of perturbative pointlike and nonperturbative hadronic (VMD) components, employing recently updated parton distributions of pions and protons. The resulting parameter-free and perturbatively stable LO and NLO parton densities f^{\gamma(P^2)}(x,Q^2) are smooth in P^2 and apply to all P^2 >= 0 whenever \gamma(P^2) is probed at scales Q^2 >> P^2 where transverse photons also dominate physically relevant cross sections. Predictions are given for the structure function F_2^{\gamma(P^2)}(x,Q^2) and f^{\gamma(P^2)}(x,Q^2), and are compared with all relevant data for real photons as well as with recent data for virtual photons as extracted from DIS ep dijet events. Simple analytic parametrizations of our predicted parton distributions are presented for the real photon in LO and NLO, and for the virtual photon in LO which, within sufficient accuracy, may be also used in NLO-QCD.

Study of the glueball candidate f J(2220) at CLEO

The CLEO detector at the Cornell e + e − storage ring CESR was used to search for the two-photon production of the glueball candidate f J(2220) in its decay to K S 0K S 0 and π + π −. I present restrictive upper limits on the product of the two-photon width and the respective branching fraction [ Γ γγ B K S 0K S 0 ] f J (2220) < 1.3 eV, 95% CL and [ Γ γγ B π + π − ] f J (2220) < 2.5 eV, 95% CL. When these two limits are combined to calculate a lower limit on the Stickiness, a measure of the two-gluon coupling relative to the two-photon coupling, a lower limit S f J (2220) > 102, 95% CL is found. A new measure of this ratio of couplings is the Gluiness; a combined lower limit of G f J (2220) > 66, 95% CL is found. These limits on the Stickiness and the Gluiness indicate that the f J (2220) is likely to have substantial neutral parton or glueball content.

Radiatively generated parton distributions of real and virtual photons

The parton content of real ${(P}^{2}=0)$ and virtual ${(P}^{2}\ensuremath{\ne}0)$ transverse photons $\ensuremath{\gamma}{(P}^{2})$ is expressed in terms of perturbative pointlike and nonperturbative hadronic (VMD) components, employing recently updated parton distributions of pions and protons. The resulting parameter-free and perturbatively stable LO and NLO parton densities ${f}^{\ensuremath{\gamma}{(P}^{2})}{(x,Q}^{2})$ are smooth in ${P}^{2}$ and apply to all ${P}^{2}&gt;~0$ whenever $\ensuremath{\gamma}{(P}^{2})$ is probed at scales ${Q}^{2}\ensuremath{\gg}{P}^{2}$ where transverse photons also dominate physically relevant cross sections. Predictions are given for the structure function ${F}_{2}^{\ensuremath{\gamma}{(P}^{2})}{(x,Q}^{2})$ and ${f}^{\ensuremath{\gamma}{(P}^{2})}{(x,Q}^{2}),$ and are compared with all relevant data for real photons as well as with recent data for virtual photons as extracted from DIS $\mathrm{ep}$ dijet events. Simple analytic parametrizations of our predicted parton distributions are presented for the real photon in LO and NLO, and for the virtual photon in LO which, within sufficient accuracy, may be also used in NLO-QCD.

Further Search for the Two-Photon Production of the Glueball CandidatefJ(2220)

The CLEOII detector at the Cornell e+ e- storage ring CESR has been used to search for the two-photon production of the $f_J(2220)$ decaying into pi+ pi-. No evidence for a signal is found in data corresponding to an integrated luminosity of 4.77/fb and a 95% CL upper limit on $\Gamma_{two-photon} * BR{pi+ pi-}$ of 2.5 eV is set. If this result is combined with the BES Collaboration's measurement of $f_J(2220) -> pi+ pi-$ in radiative $J/\psi$ decay, a 95% CL lower limit on the stickiness of the $f_J(2220)$ of 73 is obtained. If the recent CLEO result for $\Gamma_{two-photon} * BR{\K_S K_S}$ is combined with the present result, the stickiness of the $f_J(2220)$ is found to be larger than 102 at the 95% CL. These results for the stickiness (the ratio of the probabilities for two-gluon coupling and two-photon coupling) provide further support for a substantial neutral parton content in the $f_J(2220)$.

Resolved photon processes

After giving a very brief introduction to the resolved photon processes, I will summarise the latest experimental information from HERA, on resolved photon contribution to large pt jet production as well as to direct photon production. I will point out the interesting role that resolved photon processes can play in increasing our understanding of the dynamics of the Quarkonium production. I will then discuss the newer information on the parton content of virtual photons as well as the kt distribution of the partons in the photon. I will end by giving predictions of an eikonalised minijet model for siggg which crucially uses the experimental measurement of the abovementioned kt distribution and comparing them with data.

Diffractive heavy flavour production at the Tevatron and the LHC

We give predictions for diffractive heavy flavour production at the Tevatron and the LHC in leading-order approximation. In the framework of these studies we use three different models for the partonic structure of the Pomeron recently proposed by Stirling and Kunszt. These Pomeron models are, despite being fitted to the same diffractive deep inelastic HERA data, very different in their parton content and taken together provide a powerful tool to probe the structure of the Pomeron. All models satisfy GLAP evolution and show a significant Q2 dependence. We give numerical predictions for single as well as double diffractive cross sections assuming a Donnachie-Landshoff-type Pomeron flux factor.

Estimating diffractive Higgs boson production at LHC from HERA data

Using a recently proposed factorization hypothesis for semi-inclusive hard processes in QCD, one can study, in principle, the diffractive production of the Standard Model Higgs boson at LHC using only, as input, ep diffractive hard-processes data of the type recently collected and analyzed by the H1 and ZEUS collaborations at HERA. While waiting for a more precise and complete set of data, we combine here the existing data with a simple Pomeron-exchange picture and find a large spread in the Higgs boson production cross section, depending on the input parametrization of the Pomeron's parton content. In particular, if the Pomeron gluon density f g p (β) is peaked at large β for small scales, single diffractive events will represent a sizeable fraction of all produced Higgs bosons with an expected better-than-average signal-to-background ratio.

Parton content of virtual photons.

The parton content of virtual transverse photons \ensuremath{\gamma}(${\mathit{P}}^{2}$) is expressed in terms of perturbative pointlike and nonperturbative hadronic (VMD) components. The resulting LO and NLO parton distributions ${\mathit{f}}^{\ensuremath{\gamma}(\mathit{P}2}$)(x,${\mathit{Q}}^{2}$) are smooth in ${\mathit{P}}^{2}$. They apply uniformly to all ${\mathit{P}}^{2}$\ensuremath{\ge}0 whenever \ensuremath{\gamma}(${\mathit{P}}^{2}$) is probed at a scale ${\mathit{Q}}^{2}$\ensuremath{\gg}${\mathit{P}}^{2}$ where the transverse photons furthermore also dominate physically relevant cross sections. Predictions are given for ${\mathit{F}}_{2}^{\ensuremath{\gamma}(\mathit{P}2}$)(x,${\mathit{Q}}^{2}$) and ${\mathrm{\ensuremath{\mu}}}^{\ensuremath{\gamma}(\mathit{P}2}$)(x,${\mathit{Q}}^{2}$), ${\mathit{g}}^{\ensuremath{\gamma}(\mathit{P}2}$)(x,${\mathit{Q}}^{2}$) relevant for future CERN LEP and present DESY HERA measurements, respectively. Except for certain kinematic regions, these virtual structure functions and parton distributions at ${\mathrm{\ensuremath{\Lambda}}}^{2}$\ensuremath{\ll}${\mathit{P}}^{2}$\ensuremath{\ll}${\mathit{Q}}^{2}$ are not solely described by purely perturbative contributions, in contrast with ``naive'' expectations; these remaining nonperturbative components, although being based on VMD-inspired simplicity, represent the largest uncertainty in our quantitative results and eventually have to be tested by future experiments.

Virtual photon structure functions and the parton content of the electron.

We point out that in processes involving the parton content of the photon the usual effective photon approximation should be modified. The reason is that the parton content of virtual photons is logarithmically suppressed compared to real photons. We describe this suppression using several simple, physically motivated ans\"atze. Although the parton content of the electron in general no longer factorizes into an electron flux function and a photon structure function, it can still be expressed as a single integral. Numerical examples are given for the \eplem\ collider TRISTAN as well as the $ep$ collider HERA.

Diffractive W and Z production at [formula omitted] colliders and the pomeron parton content

High- p ⊥ jets in diffractive p p events have revealed a parton structure in the exchanged pomeron. Based on a model using a pomeron structure function dominated by either quarks or gluons we calculate diffractive W/ Z production and find observably large cross-sections at the Tevatron energy. The relative quark and gluon content may here be experimentally investigated and the x-shape of their density functions obtained to give new insights on the nature of the pomeron.

Study of four-jet events and evidence for double parton interactions in pp-bar collisions at sqrt s =1.8 TeV.

Kinematic properties of four-jet events produced in $p\overline{p}$ collisions at $\sqrt{s}=1.8$ TeV have been studied using data with an integrated luminosity of 325 ${\mathrm{nb}}^{\ensuremath{-}1}$ collected using the Collider Detector at Fermilab during the 1988-1989 Fermilab Collider run. The individual jet ${p}_{T}$ spectra and the angles between each jet pair are compared to the predictions of leading-order quantum chromodynamics for the double gluon bremsstrahlung process and good agreement is observed. In addition, a search for double parton scattering has been undertaken using variables sensitive to the topology of four-jet events. A small double parton content provides the best description of the data. We find $\frac{{N}_{\mathrm{DP}}}{{N}_{\mathrm{DB}}}={5.4}_{\ensuremath{-}2.0}^{+1.6}%$, where $N$ represents the number of events attributed to each process. We measure ${\ensuremath{\sigma}}_{\mathrm{DP}}={63}_{\ensuremath{-}28}^{+32}$ nb for jets having ${p}_{T}g25$ GeV/c in the pseudorapidity interval $|\ensuremath{\eta}|l3.5$.

Higher order QCD analysis of the photon structure

A new factorization scheme for the higher order QCD analysis of the photon's hadronic structure, avoiding the large-χ instabilities occuring in the usual MS treatment, is discussed. The parton content of real photons is generated in leading and higher order from a pure valence-like VMD input at a low resolution scale adopted from the pion, allowing for a prediction of the rather unknown gluon density. The agreement of the resulting quark distributions with presently available data is demonstrated. The inclusive production of heavy quarks and lepton pairs at ep colliders, which can provide separate information on the photon's gluon and quark content, respectively, is investigated.

Small-x parton content of the photon.

The parton content of the photon is examined systematically. In the low-${Q}^{2}$ region (${Q}^{2}&lt;1$ ${\mathrm{GeV}}^{2}$) we assume the photon behaves largely according to the vector-meson-dominance (VMD) hypothesis, while for higher ${Q}^{2}$ we supplement the VMD with a purely perturbative pointlike component and QCD corrections. Special care is taken in the region of small $x$: we work in the leading-logarithm approximation (LLA) in $\frac{1}{x}$ at low ${Q}^{2}$ and in the LLA in ${Q}^{2}$ for ${Q}^{2}&gt;4$ ${\mathrm{GeV}}^{2}$. We include small-$x$ shadowing effects in the VMD sector and show that they need not be considered in the pointlike sector where the parton content is much less.

Small-χ and the pomeron structure in QCD

Our present understanding of the pomeron is reviewed in the context of probing its possible parton content in diffractive hard scattering processes. A new approach is discussed where the parton density distributions of the pomeron is related to ladder and fan diagrams in perturbative QCD. In one region perturbative effects dominate and the gluon density of the pomeron is calculable leading to result s that are different from expectations in Regge theory. In another region non-perturbative effects are important, but an extrapolation of perturbative QCD gives a smooth transition to the result of Regge theory. The experimental consequences for both the ‘soft’ and ‘hard’ pomeron are discussed with emphasis on deep inelastic scattering at HERA.